1. Field of the Invention
The present invention relates to a thermal head, more particularly to a thermal head having a protective layer excellent in wear resistance.
2. Description of the Related Art
An image recording apparatus of thermal recording system is simple in constitution, which is advantageous for reducing in size and weight and lowering the rice, quiet in recording and small in power consumption, and hence it is widely used in various recording applications including a variety of printers and fax machines.
The thermal head used in such a thermal recording system comprises, for example in the case of line head, a heating unit having resistance heating elements formed in a line on an electrically insulating substrate, and electrodes connected to the resistance heating elements. The heating unit is brought into contact with recording paper direct or through a recording medium such as an ink ribbon, and recording information is sequentially entered as electric signals into each resistance heating element via electrodes, thereby recording in the principal scanning direction. At the same time the recording paper travels for recording in the sub-scanning direction, so that a two-dimensional recorded image is obtained.
As such thermal head, the structure as shown in FIG. 5 has been hitherto known. FIGS. 5A through 5C show an example of line head, FIG. 5A is a plan view, and FIGS. 5B and 5C are essential sectional views taken from plane A--A,
In a thermal head 1 shown in FIGS. 5A through 5C, reference numeral 2 denotes an electrically insulating substrate made of glazed ceramic, and a glaze layer 2b is formed on a ceramic substrate 2a. On this insulating substrate 2 are formed a resistance heating element 3 of ruthenium oxide or the like, and electrodes 4 of gold (Au) or the like, which are formed by combination of a process of forming each layer and a photolithographic process. By forming such fine patterns of electrodes 4 on the resistance heating element 3 and selectively applying an electric power between the electrodes 4, the resistance heating element 3 between the electrodes 4 can be heated in tiny dots. A protective layer 5 is formed so as to cover the resistance heating element 3 and the electrodes 4. As shown in FIG. 5B, the resistance heating element 3 and the electrodes 4 may be formed in such a manner that the electrodes 4 are formed on the resistance heating element 3 and then windows are opened in the electrodes 4 by etching so that the resistance heating element 3 between the electrodes 4 is heated in tiny dots. As also shown in FIG. 5C, reversing the order of forming and laminating, the resistance heating element 3 may be formed on the electrodes 4 in which the process of opening windows has already been conducted, so as to be heated in tiny dots.
As to the protective layer 5 in the thermal head 1 of such constitution, in order to enhance the durability, not only high wear resistance to withstand sliding contact with the recording paper is demanded, but also high smoothness of the surface, that is, small surface roughness is required so as to obtain a favorable image quality by enhancing the features with respect to contact and slip with the recording paper and so as to reduce abrasion of the layer 5. So far, as such protective layer 5, a layer of high hardness obtained by a sputtering method, CVD (chemical vapor deposition) method or other thin film forming technique has been used. However, the film forming speed is slow, and consequently it takes a long time to form the layer. Additionally a film forming apparatus is expensive, resulting in high manufacturing cost.
By contrast, as an inexpensive protective layer 5 made of glass obtained by a thick film forming technique such as printing and baking has been also used widely. The glass of this protective layer 5 contains fillers such as alumina (Al.sub.2 O.sub.3) particles in order to decrease the thermal expansion coefficient of the glass to reduce thermal stress applied to the resistance heating element, to strengthen the wear resistance of the layer 5 and to enhance the thermal conductivity.
Concerning such a protective layer 8 made of glass, for example, Japanese Unexamined Patent Publication JPA 51-56236 (1976) proposes a thermal head comprising a resistance heating element formed on a substrate and electrodes coupled to the resistance heating element, wherein at least the portion of the thermal head contacting with recording paper is coated with low melting point glass such a lead glass, and the glass layer contains a fine granular or a fibrous material such as diamond, quartz and alumina superior in wear resistance to glass. Additionally, in order to prevent oxidation of the thermal head and diffusion of the resistance heating element into the glass at the time of glass coating, it is proposed to interpose an insulating film such as SiO.sub.2 and SiO.Ta.sub.2 O.sub.5 between the resistance heating element and the glass coating and between the electrodes and the glass coating. By such coating, the wear resistance is improved without lowering the characteristics of the thermal head, so that the life of the thermal head can be extended.
Japanese Unexamined Patent Publication JPA 63-216760 (1988) proposes a thick film type thermal recording head composed by sequentially laminating a layer of thermal resistance layer and electrodes, a resistance heating element layer, and a protective layer on an insulating substrate, wherein the protective layer comprises a lower layer of alumina-filler-containing-glass, the lower layer facing the resistance heating element layer, and an upper layer of amorphous glass having an alumina filler content of zero or smaller than in the glass layer, formed at the outer side of the glass layer. As a result, the surface smoothness of the upper layer is made smaller than 0.2 .mu.m, and even if any wear, crack or flaw is formed on the glass of the upper layer, it is arrested by the glass of the lower layer and is not propagated downward, and hence there is no effect on the heating resistance element, thereby enhancing the reliability of the thermal head and extending the life time.
Japanese Unexamined Patent Publication JPA 2-292058 (1990) proposes a thick film type thermal head comprising an under-glaze-layer formed on an insulating substrate, both a heating resistance element and electrodes for energizing the heating resistance element formed on the under-glaze-layer, and an overcoat layer for covering the heating resistance element, wherein the overcoat layer is composed of a first overcoat layer formed by a thick film forming technique, and a second overcoat layer formed thereon by the thin film forming technique. The first overcoat layer is formed by printing and baking glass paste, and on the first overcoat layer is formed a second overcoat layer made of SiAlON, Ta.sub.2 O.sub.5, or SiC by a thin film forming technique such as sputtering or vacuum deposition. According to this constitution, since a material of high hardness is used in the second overcoat layer, and its surface is smooth, so that the wear resistance may be enhanced without sacrificing the printing quality.
Furthermore, Japanese Unexamined Patent Publication JPA 4-232071 (1992) discloses a thermal head comprising a glaze layer formed on a substrate, a heating resistance element layer formed on this glaze layer, a power feeding layer for feeding electric power to the heating resistance element layer, and a protective layer formed on the power feeding layer and heating resistance element layer, wherein the protective layer is made of glass containing fillers, and the mean article size of the fillers exists within a range of 1 .mu.m to 2 .mu.m. Using SiO.sub.2 --PbO--Al.sub.2 O.sub.3 --CdO as a glass material and adding 25% fillers of .alpha.--Al.sub.2 O.sub.3 having a mean particle size of 1.3 .mu.m, the surface roughness Ra.ltoreq.0.1 .mu.m can be achieved without lowering the Knoop hardness, and a thermal head having a protective layer excellent in surface smoothness and hardness can be obtained, so that printing of high quality with less paper flaw is achieved.
Japanese Unexamined Patent Publication JPA 61-229570 (1985) discloses a thermal head comprising a glazed substrate, both a heating element and an electric conductive layer provided on the glazed substrate, and a wear resistant glass layer formed thereon, wherein the wear resistant glass layer is formed on the heating element and electric conductive layer via a thin oxide film of silicon oxide, alumina or the like having a thickness of at least 300 angstroms. JPA 61-229570 also discloses that the thin oxide film of silicon oxide is formed by the CVD method, and one of alumina is formed by the sputtering method, and that lead borate glass (PbO--SiO.sub.2 --B.sub.2 O.sub.3) to which a slight amount of alumina and potassium oxide (K.sub.2 O) was added is printed and baked to form the wear resistant glass. As a result, the adhesion strength between the glass layer and the heating resistance element can be enhanced, while oxidation of the heating resistance element at the time of glass baking can be prevented, and moreover the impurity ions in the glass are prevented from diffusing into the heating resistance element to change the resistance value of a heating portion.
However, the present inventors investigated the protective layers disclosed in the above publications, and found that the following problems are still present even in the constitutions disclosed in the above publications.
That is, in the coating of JPA 51-56236, since the size of the filler is not taken into consideration, the filler may project from the surface of the coating to increase the surface roughness, namely lower the surface smoothness, resulting in lowering the printing quality, or damaging the paper. When the specific gravity of the glass is smaller than that of the filler, the filler sinks into the bottom of the coating to be distributed unevenly, and hence the wear resistance of the surface is not enhanced.
In the protective layer of JFA 4-232071, when the mean particle size of the filler is set larger than usual, since the specific gravity of the filler in comparison with the glass of the protective layer is not taken into consideration, if the specific gravity of glass is larger than that of the filler, the filler of large particle size may float to the surface of the protective layer to be distributed unevenly, resulting in increasing the surface roughness, and lowering the printing quality like JPA 51-56236. Likewise, when the specific gravity of the glass is smaller than that of the filler, the filler sinks into the bottom of the coating to be distributed unevenly the same as above, and hence the wear resistance of the surface cannot be enhanced.
Further, in the case of two-layers structure of the protective layer as disclosed in JPA 63-216760 or JPA 2-292058, although the surface of the protective layer is satisfactorily smooth, it is hard to obtain sufficient adhesion between the two layers or resistance against thermal stress, and the number of processes of protective layer fabrication increases with the result that the material and fabrication cost increases.
Moreover, as in JPA 61-229570, in the case where a thin oxide film is interposed between the heating element and the wear resistant glass layer and between the conductive layer and the wear resistant glass layer, since oxygen is present in the thin oxide film layer, sufficient oxidation preventive effect can not be attained by the heating element having a thickness not much exceeding about 300 angstroms.